Add support for highlighted text

A customer has reported an issue with text highlighting not showing.

(issue seen in IMSMO, but also applicable to Traveler)

Development have found that the Traveler server relies on the HTML API (HAPI) "HTMLGetText" to handle the conversion of a NotesRichText message to MIME. The inability to preserve highlighted text is a limitation of this API, and adding this ability would be considered an enhancement. An internal defect with the HAPI team has been raised for the request.

As a workaround until this can be addressed, development have suggested using colored text instead of highlighting, as this formatting is preserved by the HTMLConverter.